tunefs

tune up an existing file system

Changes the dynamic parameters of a file system which affect the layout policies.
cannot be run on an active file system, it must be downgraded to read-only or unmounted.

‑N

displays settable options (after any changes from the tuning options)

ommiting -A

update only standard superblock

-A

Alternate superblocks are also updated.
If a run of fsck uses an alternate superblock, which has not been updated, parameters will be those previosly written to that alternate.

-m minFree

percentage of space held back from normal users(i.e.non root); the minimum free space threshold.
Default 10%. If set to zero, up to a factor of three
in throughput will be lost over the performance obtained at a 10% threshold.
If space is allocated above the threshold, users will be unable to allocate files until enough files
have been deleted to get under the higher threshold.

This reserves space for root in the event of a rogue process or syslog logs (local or remote) attempting to consume all the space on the file system.

-d rotatDelay

expected time (in ms ) to service a transfer completion interrupt
and initiate a new transfer on the same disk. Used to decide how much rotational spacing
to place between successive blocks in a file.

-a maxContig

maximum number of contiguous blocks before forcing a rotational delay .
default is one
Device drivers that can chain several buffers together in a
single transfer should set this to the maximum chain length.

-e maxBpG

maximum number of blocks out of a cylinder group before being forced to change to another group.
Use one quarter of the blocks in a cylinder group.
preventing any file from using up all the blocks in a single cylinder group, thus
degrading access times for all files subsequently allocated in that cylinder group. The effect
of this limit is to cause big files to do long seeks more frequently than if they were allowed
to allocate all the blocks in a cylinder group before seeking elsewhere. For file systems with
exclusively large files, this parameter should be set higher.

-f avgFileSize

expected average file size.

-s avgFpDir

expected number of files per directory.

-ospace | time

Optimize for minimum space fragmentation or time spent allocating blocks.
Optimization for space has high overhead for file writes.
If minFree is less than 10%, then optimize for space to avoid running out of full sized blocks.
For of minFree greater than 10%, optimized for time.

-t trackSkew

the skew, in sectors, from one track to the next in a cylinder.
Default zero, meaining that each track in a cylinder begins at the same rotational position.